Pawn islands of either side are groups of pawns (at least one), separated by files without own pawns. Those files are therefore either halfopen or open. All other things being equal, the side with fewer pawn islands has an advantage, because the individual pawns are easier to defend against enemy attacks. Each island has a west- or east border file. Rook-files may be the most outer borders.
♟♟ ♟♟ ♟♟
Three black versus two white Islands